1. Field of the Invention
The invention relates in the first instance to a process for determining the erodability of soil, in particular soil belonging to the underwater floor, comprising installing a vertically displaceable spray nozzle above the floor, producing a water flow onto or over the floor by means of said spray nozzle, which flow forms a pit in the soil by erosion of the soil, systematicly lowering the spray nozzle by means of displacement means in a manner such that said nozzle follows the descending pit floor, determining the speed at which the spray nozzle follows the pit floor and deriving the erodability at various depths from the relationship between said speed and the flow rate of the water flow.
2. Description of the Prior Art
Such a process is known from the U.S. Pat. No. 3,690,166.
Offshore structures such as pipelines and drilling platforms are in many cases sited on fine-grained sedimentary material. Said sediment may be eroded by wave and flow action, that is to say, the grains are detached and entrained by the water. This erosion may have not only disadvantageous but also advantageous consequences. The washing away of soil near supporting pillars of platforms and the like can result in dangerous situations; on the other hand, the self-burying of pipes lying on the sea floor is an example of a desired consequence of erosion. In planning and designing structures having a foundation in the underwater floor account will therefore have to be taken of the erodability of the soil. There is therefore a need for a method and an apparatus for predicting and measuring the erodability of the soil with adequate accuracy and reproducibility. The extent to which the soil is eroded depends on the (variable) flow and wave action and on (the variation in) the soil characteristics in the horizontal and vertical direction.
Some known methods of predicting the erosion of soil exposed to wave and flow action are subject to disadvantages. They are based either on determining in situ the cone resistance, the imperviousness and the permeability of the soil, or on determining characteristics of soil samples in the laboratory, such as the composition and the grain size distribution and the erodability when exposed to a water flow. However, these methods either give only indirect low-reliability information on the erodability of the soil or the methods are very cumbersome and therefore expensive. The indirect interpretation of the data is particularly difficult if the floor contains cohesive material such as silt. This is the case in large parts of, for example, the North Sea. In addition, it is usually inadequate to examine one sample at a site, because the characteristics generally vary layer-wise and in the horizontal direction.
In the method according to said U.S. Pat. No. 3,690,166, the spray nozzle is suspended on a cable which is wound over a drum to be actuated by a motor. The tension in the cable is determined by load cells and said cells regulate the electrical energy delivered to the motor so that the cable tension remains essentially constant. Owing to differences in cable characteristics (length, stiffness) and the like this method is too inaccurate to achieve a constant distance between the spray nozzle and the floor of the small pit. Small variations in distance between the bottom of the spray nozzle and the floor of the small pit have considerable consequences in relation to the measured erodability. A constant distance is therefore necessary to obtain accurate and reproducible erodability measurement results. The known method is therefore also unsuitable in particular for determining small variations in erodability under relatively weak water flows.
The object of the invention is to avoid the disadvantages mentioned of known methods .and to provide a process of the type mentioned in the preamble with which the erodability of soil can be accurately and reproducibly determined.